As will become apparent from the discussion which follows hereinafter, photovoltaic power has not as yet attained the type of commercial acceptance which was predicted at the beginning of the decade. The reason is predominantly economic. The cost of manufacturing, distributing and servicing solar cell installations remains higher than the cost of utilizing grid supplied electrical power. In order for photovoltaically generated power to gain widespread acceptance, it is necessary for costs associated with manufacturing photovoltaic modules, as measured in terms of cost per peak watt of photogenerated power, to decrease. Manufacturing costs are dependent upon (1) the methods by which the semiconductor alloy material of the solar cells are fabricated and processed into modular format, as well as (2) the efficiency of those modularized solar cells. The assignee of the instant invention has championed the fabrication of thin film photovoltaic devices from substantially amorphous semiconductor alloy material, vis-a-vis, the more predominantly accepted fabrication of photovoltaic devices from crystalline semiconductor material. In an effort to implement said thin film semiconductor strategy, said assignee developed a high volume, continuous process in which successive layers of amorphous or substantially amorphous semiconductor alloy material could be deposited onto an elongated web in contiguous, but discrete, deposition chambers. More particularly, the layers of semiconductor alloy material thus deposited onto the web in the vacuum envelope of the deposition system could be utilized to fabricate photovoltaic devices such as single or stacked p-i-n solar cells.
It should thus become apparent that the use of such roll-to-roll processing has minimized the cost of manufacturing photovoltaic modules. Further cost reductions must therefore come by either improving the efficiency of downstream module fabrication or by increasing the photoconversion efficiency by which said photovoltaic modules convert incident radiation to electricity. The assignee of the instant invention has recently developed a method of fabricating 1 foot by 4 foot photovoltaic power modules, which modules are characterized by a reduced number of electrical connections so as to require a minimum number of processing steps. These high current, low voltage power modules have, for the first time, provided for the profitable sale of photovoltaically generated electricity at a cost of about four dollars per peak watt. Therefore, the instant inventors believe that a significant effort in reducing the cost per peak watt of photovoltaically generated electricity has been made in module fabrication and additional reductions must be derived from further improvements in the photoconversion efficiency of said modules.
It is to be noted that the assignee of the instant invention has achieved the highest reported thin film solar cell photoconversion efficiency, an efficiency of about thirteen percent. This record was achieved utilizing a photovoltaic structure in which three discrete p-i-n type solar cells were optically and electrically stacked in series, each cell dedicated to the absorption of photons of incident radiation of a particular portion of the solar spectrum. Through this "spectrum splitting" technique, it becomes possible to fabricate the stacked photovoltaic structure with a plurality of relatively thin photogenerative layers so that the built-in electric field provided by the doped layers effectively collect photogenerated charge carriers and thereby minimize recombinative losses due to "Staebler/Wronski" degradation. The manner in which stacked cells operate to minimize recombinative losses will be described in greater detail in a later portion of this Background.
The concept of utilizing multiple stacked cells to enhance photovoltaic device efficiency was described as early as 1955 by E. D. Jackson in U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structures disclosed therein were limited to the utilization of p-n junctions formed by single crystalline semiconductor devices. Essentially, the stacked cell concept employs different band gap devices to more efficiently collect various portions of the solar spectrum and to increase V.sub.oc (open circuit voltage). In the uppermost or light incident cell, a relatively large band gap semiconductor material absorbs only the short, highly energetic wavelength light; while in the subsequent cells, subsequently smaller band gap materials absorb the longer, less energetic wavelengths of light which pass through the first cell. By substantially matching the photogenerated currents from each serially connected cell, the overall open circuit voltage becomes the sum of the open circuit voltage of each cell; while the short circuit current (J.sub.sc) of each cell remains substantially constant. Such tandem structures are now commercially employed by the assignee of the instant invention in the large area photovoltaic devices referred to hereinabove by utilizing the aforementioned continuous processing techniques for depositing successive thin film layers of amorphous and microcrystalline semiconductor alloy materials.
In the description which follows, it is to be kept in mind that specialized definitions of amorphicity and microcrystallinity are employed. The term "amorphous", as used herein, is defined to include alloys or materials exhibiting long range disorder, although those alloys or materials may exhibit short or intermediate range order or even contain crystalline inclusions. The term "microcrystalline", as used herein, is defined as a unique class of said amorphous materials characterized by a volume fraction of crystalline inclusions, said volume fraction being greater than a threshold value at which the onset of substantial changes in certain key parameters such as electrical conductivity, optical band gap and absorption constant occurs. It is to be specifically noted that pursuant to the foregoing definitions, microcrystalline material falls within the generic class of amorphous materials.
The assignee of the instant invention has also been active in the development of (1) improved semiconductor alloy materials, both wide and more narrow band gap materials, which materials are characterized by a reduced density of defect states (as low as about 10.sup.16 cm.sup.-3 eV.sup.-3); (2) improved back reflector materials, including dual layer, highly reflective materials (such as silver buffered by a layer of zinc oxide); and (3) improved, wide band gap doped layers of semiconductor alloy material characterized by high conductivity for increasing the built-in electric field of the solar cells in which they are incorporated. All of these developments were essential in order to develop the aforementioned triple stacked photovoltaic device exhibiting the world record 13% photoconversion efficiency. However, as described hereinabove, even that world record efficiency remains insufficient to provide for cost effective competition between photovoltaically generated electricity and electricity derived from more conventional, depletable energy sources.
It was in an effort to further increase the photovoltaic conversion efficiency of solar cells that the development work which led to the improved cell performance occasioned by the instant invention was undertaken. The strategy employed by the instant inventors was to return to basic considerations which impact upon thin film solar cell design, which considerations are currently accepted by researchers in the field and to reexamine the "conventional wisdom" or given truths regarding the operational interactions of the multilayers thereof. Said inventors were particularly interested in examining the physics of operation of the intrinsic layer of semiconductor alloy material of single and tandem solar cells so as to find a means for enhancing the open circuit voltage derived therefrom without sacrificing the efficient collection of photogenerated charge carriers. It should be noted as a base-line reference, that photovoltaic design had heretofore required the presence of a homogeneous thin film layer of amorphous silicon alloy material (for 1.7 eV optical band gap material) or amorphous silicon germanium alloy material (for less than 1.7 eV optical band gap material) sandwiched between layers of p and n-type semiconductor alloy material. Prior to summarizing the inventive concept disclosed herein, it will be helpful to detail the efforts of researchers in the field to improve solar cell efficiency by modifying the homogeneous nature of the intrinsic layer of semiconductor alloy material represented by said "base-line."
In the course of this analysis, the instant inventors revisited the work of others in the field of solar cell fabrication to consider the type of "unusual" intrinsic layer designs which had previously been considered. For instance, a photovoltaic device constructed with a varying band gap in a narrow portion of the intrinsic layer is disclosed in a paper entitled "Achievement of Higher Efficiency Amorphous Silicon-Germanium Solar Cells Using Affinity Gradients" presented by S. Wiedeman and E. A. Fagen at the 17th Annual I.E.E.E. Photovoltaic Conference held May 1-4, 1984 in Kissimmee, Fla. Disclosed therein is a n-i-p-type photovoltaic device formed of an amorphous silicon-germanium alloy in which the composition of the intrinsic layer of semiconductor alloy material was profiled over the first few hundred angstroms from the light incident surface thereof. This band gap variation was accomplished by gradually altering the ratio of silicon to germanium in those few hundred angstroms. The object of such band gap variation was to establish an electrical field of varying strength adjacent the light incident surface of the intrinsic layer of semiconductor alloy material, which field was adapted to decrease if not eliminate charge carrier losses at the interface of the n type and intrinsic layer. Such losses were occasioned by the back diffusion of charge carriers across the n layer and intrinsic layer interface. The authors of the paper claimed that, because of the presence of the electric field, a 29% improvement in the initial photoconversion efficiency of the photovoltaic devices was achieved.
In commonly assigned U.S. Pat. No. 4,547,621 entitled "Stable Photovoltaic Devices And Method of Producing Same" (the disclosure of which is incorporated herein by reference), M. Hack and S. Guha graded the band gap of the intrinsic layer of silicon alloy material of a light incident n-i-p type photovoltaic device such that the wider band gap portion was disposed proximate the light incident surface. The wide band gap portion (a) was formed so as to be less than one half the thickness of the remaining narrower band gap portion and (b) included at least one band gap broadening element not present in the narrower band gap portion. In this manner, the authors attempted to provide for the uniform absorption of photons of short, highly energetic incident radiation throughout at least a substantial portion of the bulk of the intrinsic layer so as to promote the photogeneration of electron-hole pairs throughout said substantial portion of the intrinsic layer and thereby reduce charge carrier recombination therein. In this manner, the authors claimed to improve long term stability.
In commonly assigned U.S. Pat. No. 4,379,943 entitled "Current Enhanced Photovoltaic Device" (the disclosure of which is incorporated herein by reference), C. Yang, A. Madan, S. Ovshinsky and D. Adler disclosed the fabrication of a novel photovoltaic structure in which the intrinsic layer of semiconductor alloy material includes a first intrinsic layer formed of a non-etching (non-fluorinated) precursor gaseous mixture and a second intrinsic layer preferably formed of silicon and fluorine. The thicknesses of the first and second intrinsic layers were adjusted to match the respective potential drops thereof with the first intrinsic layer being relatively thin and the second intrinsic layer being relatively thick. The short circuit current of the photovoltaic device was said to be enhanced because the first and the second intrinsic layers are fabricated with differing band gaps so as to provide a field throughout those layers.
In commonly assigned U.S. Pat. No. 4,471,155 entitled "Narrow Band Gap Photovoltaic Devices With Enhanced Open Circuit Voltage" (the disclosure of which is incorporated herein by reference), R. Mohr and V. Cannella designed a photovoltaic device which provided enhanced open circuit voltage by fabricating the narrow band gap intrinsic semiconductor alloy material thereof so as to include a second intrinsic region having a band gap wider than the band gap of the first intrinsic region. The second band gap region is disposed between the first band gap region and one of the doped layers. This open circuit enhancement structure can also include a third intrinsic region, which region has a wider band gap than the first intrinsic region and is disposed on the side of the first intrinsic region opposite the second intrinsic region. As is explicitly stated in paragraph 3, lines 18-26 of this patent, "One effect of the increased density of defect states in amorphous silicon-germanium alloys is the reduction in open circuit voltage by an amount larger than can be reasonably expected by the reduction in band gap. This reduction in voltage is attributed to increased recombination at the increased defect states, and to interface states introduced by band gap and structural mismatch at the doped region-intrinsic region boundary." (The emphasis was added because the underlined quote inferentially indicates that [as indicated hereinabove] prior to the instant invention, it was commonly accepted that open circuit voltage delivered by a given cell was limited by the band gap of that cell).
The review of thin film solar cell design undertaken by the instant inventors also located patents which disclosed solar cells wherein the intrinsic layers of semiconductor alloy material were graded substantially throughout the bulk thickness thereof. Specifically relevant were two patents of S. Yamazaki; the first being U.S. Pat. No. 4,239,554 entitled "Semiconductor Photoelectric Conversion Device" and the second being U.S. Pat. No. 4,254,429 entitled "Hetero Junction Semiconductor Devices." It is to be noted that the disclosure of the Yamazaki patents ascribe the inventor's motivation in grading the intrinsic layer was to eliminate the notch or spike which can be formed at the heterojunction (defined, for purposes of this description, as the junction between two regions of the intrinsic material characterized by different band gaps), said spike being due to the absorption of holes or electrons moving from one of two adjacent semiconductor regions across the heterojunction and into the other region. It is to be noted, however, that the reason for such gradation in the Yamazaki patents was to provide for charge carrier transport from the wide band gap energy of one of the doped layers to the narrow band gap energy of the oppositely doped layer without interposing a spike which would prevent or at least interfere with charge carrier mobility. Thus, Yamazaki noted that such transport could most easily be effected by a smooth, continuous band gap variation through the intrinsic layer.
Finally, the instant inventors also became aware of a SERI (Solar Energy Research Institute) report tendered by V. Dalal of Spire in which the band gap of the intrinsic layer of an amorphous silicon alloy solar cell was smoothly graded to achieve a higher open circuit voltage. Dalal proceeded to hypothesize that it would be possible to obtain a higher fill factor by implementing a graded structure so as to initiate an increased drift field which promotes electron trasport in the graded regions. However, the disclosed SERI report failed to achieve significant improvements in V.sub.oc, incorrectly moved the conduction band so as to improve electron transport (although hole transport represents the limiting charge carrier collection factor in amorphous silicon alloy cells) and utilized doped layers, at least one which was characterized by a band gap less than the band gap of the intrinsic layer.
With this analysis of the prior art in intrinsic layer design as a backdrop and once again realizing that a multijunction approach to the development of high efficiency and stable amorphous silicon alloy solar cells will yield the best results, the critical solar cell design considerations for the fabrication of intrinsic layers of single as well as for tandem photovoltaic structures can now be enumerated. In this approach, the intrinsic layer of each discrete cell is relatively thin so that the photogenerated charge carriers have a relatively short distance to travel before reaching the respective electrodes thereof. At the same time, by stacking a plurality of cells in optical and electrical series relation, all of the photons of the incident solar spectrum are absorbed with discrete cells dedicated to the absorption of a particular range of wavelengths. Since the current photogenerated by each cell must be matched, the top cell is made thin and therefore exhibits stability against the degrading effects of light exposure. The lower cells receive progressively less intense illumination and due to the fact that a cell receiving light of less intensity over a longer period of time does not degrade as much as a cell receiving light of greater intensity over a shorter period of time; the lower cells can be made progressively thicker without jeopordizing stability.
The p-type and n-type layers provide the internal electric field across the intrinsic (photogenerative) layer of a solar cell. It is important that the doped layers are characterized by the highest possible conductivity so that there is no rectifying internal junction formed between the p and n layers of contiguous cells. Further, and of equal importance, high conductivity p and n layers increase the built-in electric field across the photogenerative intrinsic layer, thereby increasing the open circuit voltage and fill factor of the cell. It is of substantial importance that the p and n layers be fabricated of wide band semiconductor alloy materials (certainly wider than the band gaps of the intrinsic layers) so as to exhibit little or no optical absorption of photogenerative photons of incident radiation (charge carriers generated in the doped layers will instantaneously recombine).
It is additionally necessary that the narrow band gap semiconductor alloy material be of as high a quality (possess as low a density of defect states) as possible. Of course, as the band gap of the amorphous silicon alloy material is lowered through the addition of tin, or preferably germanium, additional defect states are created. This is due to (1) the preferential attachment of hydrogen atoms to silicon, thereby creating dangling bonds of germanium in the alloy; (2) the tendency of germanium to assume divalent configurations, thereby initiating additional defect states; and (3) the tendency of non-crystalline germanium films to grow in a columnar fashion, thereby degrading film quality. As previously mentioned, the assignee of the instant invention has, through the incorporation of fluorine into the narrow band gap material, effectively lowered the density of defect states and the degree of sub-band gap absorption in the material without changing the slope of the valence band tail as the optical band gap is decreased to a value as low as about 1.25 eV.